The present invention generally relates to a process for making granulate of polymeric material.
The publication, entitled "Spin-Away-Dryer Zentrifugaltrockner zum Entwassern von Kunststoffgranulaten", by G. Gotz, GAK 12/1972, page 1180, discloses a typical granulating plant for making granulate of polymeric material by pressing strands, formed from a plastic mass, in a stream of cooling liquid, e.g. water, and shredding the plastic strands in the cooling liquid into granulate, thereby forming a dispersion. This dispersion of granulate and cooling liquid is conveyed to a separator to increase the amount of granulate in the cooling liquid by removal of a portion of cooling liquid. The so-concentrated dispersion of granulate and cooling liquid is subsequently conveyed to a dryer for separation of the cooling liquid from the granulate. The dryer is situated, i.a. for space-saving reasons, in immediate proximity of the granulating device. Positioned immediately upstream of the dryer is a pre-separator which separates just enough water so that the remaining amount of water still ensures a safe transport of the granulate stream from the pre-separator into the dryer while providing relief for the dryer. Thus, the distance between the pre-separator and the dryer is bridged by a respectively short, and moreover descending conduit which has a same diameter as the conduit between the granulating device and the pre-separator. With this conventional process, the transport of the dispersion of granulate and cooling liquid, exiting the granulating device, to a dryer becomes difficult when the dryer is located at a remote location.
According to a brochure released by the company GALA Kunststoff-und Kautschukmaschinen GmbH, a process is known in which powdery or coarse polymeric material is heated by an extruder, plastifyed, and pressed through a perforated plate into a stream of water, thereby forming single strands of the plastic mass. Placed in the stream of water is a blade which rotates to shred the solidified singled plastic strands into granulate. The dispersion of cooling granulate and water is conveyed hydraulically along a relatively short path in a feed conduit to a dryer by which water is separated from the granulate. The separated water is then filtered, tempered and returned under pressure by a pump back into a reservoir, while dried granulate is transported pneumatically to a relatively remote location, for example, to a storage silo.
For technological reasons, this conventional process must be operated with a water amount which is high compared to the amount of granulate, i.a. in order to prevent formation of so-called double grain as a result of bonded granules due to insufficient cooling action. Therefore, the granulate content of the dispersion in the conduit to the dryer ranges only about 5 to 10% by volume. Such a low granulate content does not permit to operate a hydraulic conduit in a cost-efficient manner. For that reason, the dryer is positioned in proximity of the extruder, and the dried granulate is conveyed, in case of need, pneumatically. A pneumatic conveyance is, however, fairly complicated.
From the dissertation, entitled "Hydraulische Forderung von Eis zum Kuhlen von Bergwerken", Technical University of Hannover, 1991, relating to a different field of art, a test plant is known for hydraulically conveying a mixture of ice and water. The test plant includes essentially a ring-shaped closed pipeline which is supplied from a vacuum-ice machine with a dispersion of ice and water, with the content of ice being relatively low. By means of a centrifugal pump, the ice content is increased by pumping the dispersion at low flow rate in a circular path, while a volume flow of water is withdrawn from the pipeline via an integrated screen pipe whereby the volume flow of water is equal to the volume flow of the ice/water dispersion fed from the vacuum-ice machine, and is returned to the vacuum-ice machine. When the desired ice content is reached, the pump output is increased until the desired flow velocity for the test is realized. The increase of the ice content of the dispersion is however rather limited because the output of a centrifugal pump progressively deteriorates as the ice content, i.e. solids content, rises.